Several conveyor systems of the above mentioned type are known, for example from U.S. Pat. No. 3,636,883. With such conveyor carriages that cannot be lifted off from the rails, that the driving wheel is active at all times. Depending on the type of gear mechanism, the resistance in a driving wheel can range from infinity in the case of a self-locking gear, to nearly zero in the case where no transmission gear is present between the motor and the driving wheel. Accordingly, depending on the construction of the gear mechanism, the driving wheel will exert a variable resistance against a manual displacement, so that movement of the conveyor carriages within the rails may be either impossible or difficult.